gistfile1.php

<?php

namespace Drupal\bartik\blocks;

use Drupal\layout\Plugin\block\block\PagePrimaryLinks;

class PrimaryLinks extends PagePrimaryLinks {

  /**
   * Implements BlockInterface::build().
   */
  public function build() {
    return array(
      '#prefix' => '<div id="main-menu" class="navigation">',
      '#suffix' => '</div>',
      '#children' => theme('links__system_main_menu', array(
        'links' => menu_main_menu(),
        'attributes' => array(
          'id' => 'main-menu-links',
          'class' => array('links', 'clearfix'),
        ),
        'heading' => array(
          'text' => t('Main menu'),
          'level' => 'h2',
          'class' => array('element-invisible'),
        ),
      )),
    );
  }
}